﻿using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Text;

namespace Blogs.Helper
{
    #region 注释勿看
    //public static class AddLinq
    //{
    //    //public static Expression<Func<T, bool>> True<T>() { return f => true; }
    //    //public static Expression<Func<T, bool>> False<T>() { return f => false; }

    //    //public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1,
    //    //                                                    Expression<Func<T, bool>> expr2)
    //    //{
    //    //    var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
    //    //    return Expression.Lambda<Func<T, bool>>
    //    //          (Expression.Or(expr1.Body, invokedExpr), expr1.Parameters);
    //    //}

    //    //public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1,
    //    //                                                     Expression<Func<T, bool>> expr2)
    //    //{
    //    //    var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
    //    //    return Expression.Lambda<Func<T, bool>>
    //    //          (Expression.And(expr1.Body, invokedExpr), expr1.Parameters);
    //    //}

    //} 
    #endregion
    public static class AddLinq
    {
        public static Expression<Func<T, bool>> True<T>() { return f => true; }
        public static Expression<Func<T, bool>> False<T>() { return f => false; }
        public static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
        {
            // build parameter map (from parameters of second to parameters of first)  
            var map = first.Parameters.Select((f, i) => new { f, s = second.Parameters[i] }).ToDictionary(p => p.s, p => p.f);

            // replace parameters in the second lambda expression with parameters from the first  
            var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);

            // apply composition of lambda expression bodies to parameters from the first expression   
            return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
        }

        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
        {
            return first.Compose(second, Expression.And);
        }

        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
        {
            return first.Compose(second, Expression.Or);
        }
    }
    public class ParameterRebinder : ExpressionVisitor
    {
        private readonly Dictionary<ParameterExpression, ParameterExpression> map;

        public ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
        {
            this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
        }

        public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
        {
            return new ParameterRebinder(map).Visit(exp);
        }

        protected override Expression VisitParameter(ParameterExpression p)
        {
            ParameterExpression replacement;
            if (map.TryGetValue(p, out replacement))
            {
                p = replacement;
            }
            return base.VisitParameter(p);
        }
    }
}
